Vapor deposition, in particular physical vapor deposition (PVD) and chemical vapor deposition (CVD), comprises low pressure processes to condense and deposit vaporized material onto workpiece surfaces. PVD involves purely physical processes such as high temperature vacuum evaporation followed by condensation, or plasma sputter bombardment.
CVD employs chemical reactions to produce high-purity, high-performance solid materials. The process is used in semiconductor manufacturing to form thin films. Substrates are exposed to one or more vaporized chemicals which react or decompose on the target surface to produce the desired deposit. Often, volatile by-products are produced. These are removed from the reaction chamber by carrier or reaction gases.
Atomic layer deposition (ALD) is a chemical vapor-based method suited to form conformal films on both planar and complex structures. ALD is based on sequential and self-limiting surface reactions, whereby thin film deposition is controlled by surface chemistry at the atomic scale. ALD typically requires two chemically selective half-reactions utilizing vapor phase chemical species—often an organometallic precursor and a co-reactant such as water, ammonia, or hydrogen sulfide—separated in time or space by an inert gas purge.
During the first half-reaction, a first chemical precursor is introduced into the reaction chamber and reacts with surface species, changing the initial surface termination chemistry, perhaps to contain first precursor moieties. This self-limiting aspect prevents further deposition of the first precursor.
Following a purge step to remove remnants of the first half reaction, a second chemical precursor or reactant contacts the surface until all reactive surface sites have been terminated with the new species. The reaction chamber is again purged to remove unreacted precursors and products. The process is repeated if necessary to attain the predetermined thickness of the film desired
One benefit of the self-limiting nature of ALD growth is uniform deposition largely independent of chemical precursor exposure time. This means that even large and long reactant doses—necessary to reach and react with the most distant sites on convoluted, high surface area structures—result in conformal and pinhole-free growth without excess build-up of material on any one region. These features, combined with low growth temperatures and medium vacuum processing, make ALD a suitable vehicle to prepare high quality films of materials such as oxides, nitrides, sulfides, and even pure elements. However, there exists a need for in situ measurements, in particular mass and thickness measurements, to more thoroughly understand the process and rapidly advance the ALD art.
Obtaining accurate measurements of tiny changes in mass during ALD is difficult because state of the art microbalances, such as those utilizing oscillating crystal paradigms, are intended to be operated at room temperature. For example, AT-cut quartz crystals (utilized in quartz crystal microbalances or QCMs) are extremely sensitive to temperature fluctuations when operated at temperatures exceeding 50° C. AT-cut crystals, typically used in sensor applications, comprise quartz blanks formed from a thin plate cut at an angle of about 35° 15′ to the optic axis of the crystal. At the intended operating temperature for the AT-cut crystal of 25 Hz/° C. C, the temperature coefficient is 0 Hz/° C. However, at a typical ALD temperature of about 180° C., the temperature coefficient for the AT-cut quartz crystal is 50 Hz/° C. Therefore, a 1° C. temperature increase will result in an apparent change in the ALD Al2O3 thickness of minus 17 Angstroms (Å), which represents more than 15 ALD coating cycles.
Tight temperature control, e.g., with PID controllers to reduce temperature-induced variations from the mass balance, may not always be practical. Potentials for error increase with increasing temperature, reaching more than 200 Å/° C. under some ALD-relevant conditions. Moreover, thermal transients induced by the ALD precursor exposures cannot be eliminated even using PID temperature controllers.
To minimize the temperature effects of QCM measurements during ALD, another crystal, e.g., a GaPO4 sensor, can be used instead of the traditional AT-cut quartz crystal. Over the range of typical ALD processes (25-400° C.) the temperature coefficient of the GaPO4 sensor is on average, much lower. However, the temperature sensitivity of GaPO4 systems at temperatures less than 150° C. is greater than AT-cut quartz.
Accurately estimating the mass deposited during ALD is further complicated by deposition on both the front and back of the oscillating crystal—an inevitable consequence of the self-limiting surface chemistry.
Several shortcomings limit the utility of oscillating crystal systems for ALD. First, temperature equilibration of conventional QCM sensor heads (i.e., fixture on a stick configurations) requires several hours. This, combined with a significant disturbance of the gas flow dynamics produced by conventional QCM sensor heads, precludes the technique as a routine in situ “monitor” of thickness for co-deposited samples of interest.
Given the growing efforts to develop more effective ALD processes and materials, there exists a need for in situ characterization during regular/normal operation. This in situ characterization should be seamlessly integrated with the ALD equipment for convenience and high throughput. The measurement system should accommodate variations in temperature and process conditions to provide an accurate measurement of the ALD process, and particularly to provide an accurate characterization of films as they are being fabricated. This will allow film thicknesses to be tuned during their fabrication and also allow films to be characterized at several locations of the film surface simultaneously. The system should also provide rapid temperature equilibration after substrate loading, and present a minimal disturbance to the ALD environment. Finally, the system should be easily implemented in a broad variety of ALD equipment platforms having different sizes, shapes, and configurations.